The OLEDS are electronic devices that emit light when applied a voltage. Tang et al. of the Kodak Company were the first to disclose in 1987, in “Applied Physics Letters”, and in 1989 in the “Journal of Applied Physics” OLEDs having high luminance efficiency. Since then, numerous OLED structures with improved characteristics, including those using polymers, have been described.
Such an electronic device, emitting light downward (100), through a transparent substrate (140), is described in FIG. 1. The device includes a lower transparent electrode (130), an organic electroluminescent structure (120) in which an electron-hole type of conduction can take place, and a reflective metallic top electrode (110). Most often the organic electroluminescent structure itself is layered and may include a hole injection layer (129), a hole transport layer (127), a light emitting layer (125) produced by the recombination of holes and electrons, an electron transport layer (123) and an electron injection layer (121). The lower transparent electrode (130) is the anode of the device while the upper reflective metal electrode (110) is the cathode.
The luminance output efficiency is one of the important criteria used to characterize an OLED. It determines how much current or power must be supplied to have OLED delivering a given level of light in output. As the lifespan of an OLED is inversely proportional to its operating current this means that the device having a better efficiency will be able of delivering an identical level of light for a longer period of time.
One of the key parameters that limit the luminance output efficiency of an OLED is the output coupling of the photons generated by the recombination of electrons and holes, i.e., the proportion of photons that can actually escape from the device. Because of the rather high optical indexes of the materials used to form the organic layers and the transparent electrode, most of the photons generated by the recombination process are trapped by internal reflections. They can not leave the device and provide no effective contribution to the output light. Currently, up to 80% of light produced can be lost in this way.
A known method to improve the luminance output efficiency consists in forming a microcavity (MC) in the layered structure as illustrated in FIG. 1b (150). Such a configuration often called MC-OLED is described for example in U.S. Pat. No. 6,406,801. The organic electroluminescent layer (170) is placed between two highly reflective mirrors. The upper mirror is the metallic cathode (160) while the lower mirror (190) is made of alternating layers of multiple materials having different refractive indices (such as oxides of silicon and titanium). This type of multilayered mirror, also called Bragg reflector, is however designed to also transmit light. It is semi transparent to let produced light get out of the device. In this structure the anode (180) is made of an oxide of indium and tin (ITO), which is a transparent conductor material often used for the anodes of OLEDs. Reflective mirrors form a Fabry-Perot resonator, which strongly modifies the emissive properties of the organic electroluminescent structure inserted in the microcavity. The emission of light near the resonance wavelength of the cavity is strengthened while other wavelengths are eliminated or significantly reduced. The use of a microcavity in an OLED is described for example in U.S. Pat. No. 6,326,224 in order to reduce the bandwidth of the emitted light thus improving its color purity.
In structures such as the one described in FIG. 1b, a major obstacle to obtaining OLEDs having optimal performances is that the anode, made of ITO, is a rather poor conductor of electricity when compared, for example, to the metallic cathode. It does not allow to get a very low sheet resistance (Ω/cm2) (181) unless to have a very thick anode which is not possible however without directly impacting the optical properties of the microcavity formed between the upper metal layer, the cathode, and a reflecting lower mirror situated under the anode. In this structure, the optimization of the microcavity optical parameters (optical length) is not independent of the electrical parameters. Thus, the injection of current in the anode to get the specified brightness level generally causes thermal heating which is very detrimental for the lifespan of the organic layers situated immediately above. In addition, a significant voltage drop can be seen along the section of the device that does not permit to obtain identical operational characteristics over the whole surface. Finally, it should be noted that ITO is an expensive material.
Another drawback of the structure shown in FIG. 1b is the complexity of the multilayered lower mirror. To be fabricated it requires the deposition of multiple layers of materials alternating different refractive indexes with a technique known as sputtering.
In order to overcome the above problems, the replacement of the anode made of ITO by a simple semi-transparent metal layer has recently been experimented and results published by Peng et al. in “Applied Physics Letters 87”, 173505, 2005 in a paper untitled: “Efficient organic light-emitting diode using semitransparent silver as anode”. The structure disclosed in this publication indeed gets rid of the complex lower multilayered mirror since the semi transparent metallic anode also acts as a mirror to create the microcavity with the upper metallic electrode.
Although silver (Ag) has the highest electrical conductivity among all metals commonly used by the microelectronics industry, is less expensive than ITO, has excellent optical properties (low absorption over the range of visible light) and can simply be deposited by thermal vacuum evaporation, it has also, unfortunately, a relatively low work function which is becoming a barrier for the injection of holes in the organic electroluminescent layer. The barrier induced an increase of the voltage necessary to obtain the proper functioning conditions for the device.
Indeed, the performances reported in the above publication on OLEDs using a straight deposition of Ag to form the anode are worse than those using ITO. For best results the publication states that deposited Ag must be processed in a CF4 plasma to create a thin top layer (CFx) in order to reduce significantly the above mentioned barrier that otherwise prevents the effective injection of holes in the organic layer. These improved results are however obtained at the expense of the introduction of a new processing step (Ag treatment in a CF4 plasma), which further complicates the fabrication process, thus prompting to turn away from this solution.
In the field of monochromatic OLEDs other improvements have also been reported like in patent application US 2007/0001570 published by USPTO (United States patent and trademark office) on Jan. 4, 2007. The application states that color purity can be improved in a MC-OLED without impairing luminance efficiency. Notably, application discloses a bottom light-emitting OLED structure which however still requires the use of an ITO compound (ITSO) and sputtering techniques both for the deposition of ITSO and Aluminum (Al) used for the upper metal electrode. Sputtering is however a very disturbing and too energetic processing step to carry out over the already deposited organic layers of the OLED.
Therefore, in view of the foregoing, there is a need for a diode structure and a simple fabrication process of organic light emitting diodes which however permit to produce devices having very high luminance output efficiency. Especially, the process must be such that colors other than green, for which the best experimental results in terms of luminance efficiency have already been reported so far, are also improved.
Thus, it is the prime object of the invention to describe a method to obtain a bottom-emitting OLED that essentially requires simple vacuum deposition of layers of organic and metallic materials by thermal evaporation.
It is also an object of the invention to allow optical and electrical parameters to be independently adjusted so as to obtain the best possible luminance efficiency.
It is still another object of the invention to allow luminance efficiency to be independent of the physical dimensions of the devices so that they can be produced at whichever required scaling factor.
It is more specifically an object of the invention to describe a fabrication process of an OLED and an OLED device using a semi-reflective metallic anode emitting in the red with luminance efficiency greater than the experimental results reported so far.
Further objects, features and advantages of the present invention will become apparent to the ones skilled in the art upon examination of the following description in reference to the accompanying drawings. It is intended that any additional advantages be incorporated herein.